Add a unique "eid" value to all zevents

Tagging each zevent with a unique monotonically increasing EID
(Event IDentifier) provides the required infrastructure for a user
space daemon to reliably process zevents.  By writing the EID to
persistent storage the daemon can safely resume where it left off
in the event stream when it's restarted.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Issue #2

include/sys/fm/util.h

@@ -81,6 +81,7 @@ typedef struct zevent_s {
 	list_t		ev_ze_list;	/* " */
 	list_node_t	ev_node;	/* " */
 	zevent_cb_t	*ev_cb;		/* " */
+	uint64_t	ev_eid;
 } zevent_t;
 
 typedef struct zfs_zevent {